1. Field of the Invention
The field of the invention relates to internal combustion engines having vacuum assisted brake systems.
2. Background Art
Internal combustion engines are known that improve thermal efficiency and reduce pumping work by operating lean of stoichiometry and increasing manifold pressure. It is also known to extend lean operation by performing stratified operation where fuel is injected directly into the engine cylinder during a compression stroke. These engines are also capable of performing homogeneous operation where fuel is injected during an intake, or suction, stroke. Typically, stratified operation is limited to lean air/fuel ratios, while homogeneous operation can be both lean and rich of stoichiometry.
Vehicle brake systems are also known that use vacuum assist to increase driver braking force. In these systems, vacuum generated by engine operation provides extra force to assist driver braking. When these braking systems are combined with lean burn engines, engine operation can be controlled so that desired vacuum is supplied during braking. One approach uses a vacuum pressure sensor located in a brake booster to indicate available braking vacuum. When available vacuum falls below a predetermined value, engine air/fuel ratio is decreased toward stoichiometry and stratified operation is discontinued. Vacuum sensor degradation is determined when sensor voltage is outside a predetermined range of acceptable limits. Such a system is described in U.S. Pat. No. 5,826,559.
The inventors herein have recognized a disadvantage with the above approach. In particular, in-range sensor degradation is not addressed in the above system. For example, if the sensor indicates insufficient vacuum when sufficient vacuum is present, the engine air/fuel ratio is unnecessarily decreased and fuel economy can be degraded. In other words, the prior art does not monitor such a situation.
An object of the present invention is to determine degradation of a vacuum sensor coupled to a vacuum brake booster.
The above object is achieved and disadvantages of prior approaches overcome by a method for determining operability of a sensor in a vacuum brake booster coupled through a check valve to a manifold of an internal combustion engine, the method comprising: measuring or estimating an engine or vehicle operating parameter; and determining degradation in the brake booster sensor based on said operating parameter.
By using other operating parameters as an indication of operation, it is possible to determine in-range sensor degradation. In other words, system redundancy is exploited to extract information provided by other engine and/or vehicle sensors to determine in-range sensor degradation.
An advantage of the above aspect of the present invention is that improved fuel economy can be achieved by more accurately selecting desired engine combustion modes.
In another aspect of the present invention, the above object is achieved and disadvantes of prior approaches overcome by a method for determining oprability of a sensor in a vacuum brake booster coupled through a check valve to a manifold of an internal combustion engine in a vehicle system, the method comprising: determining that a braking cycle has been completed based on vehicle infomration; and determing degradation in the sensor when the sensor value changes by less than a predetermined amount during said braking cycle.
By using vehicle information to determine a braking cycle, it is possible to determine that brake booster pressure should have changed. When a change is not detected, degradation can be determined. Thus, from the vehicle information, it is possible to determine in-range sensor degradation.
An advantage of the above aspect of the present invention is that improved fuel economy can be achieved by more accurately selecting desired engine combustion modes.
In yet another aspect of the present invention, the above object is achieved and disadvantages of prior approaches overcome by a method for determining operability of a sensor in a brake booster coupled through a check valve to a manifold of an internal combustion engine, the sensor measuring a pressure in the brake booster, the method comprising: calculating a manifold pressure; and determining degradation in the sensor when the brake booster pressure is greater than said manifold pressure by a predetermined amount. By comparing manifold pressure to brake booster pressure, degradation can be determined when brake booster pressure is greater than manifold pressure. In other words, since the check valve prevents flow from entering the brake booster and only allows flow to exit the brake booster when manifold pressure is less than brake booster pressure, when brake booster pressure is indicated to be greater than manifold pressure, in-range sensor degradation can be detected.
An advantage of the above aspect of the present invention is that improved fuel economy can be achieved by more accurately selecting desired engine combustion modes.
Another advantage of the above aspect of the present invention is that degradation can be detected at many times during vehicle operation.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.